Process of treating magnesium-bearing aluminum base alloys with boron trifluoride



llnited States Patent PROCESS OF TREATING MAGNESIUM-BEARING ALUMINUM BASE ALLOYS WITH BORON TRI- FLUORIDE Spencer R. Milliken, Lower Burrell Township, Westmoreland County, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application March 26, 1958 Serial No. 723,989

8 Claims. (Cl. 14813.1)

This invention relates to the thermal treatment of articles of aluminum base alloys containing magnesium. More particularly, it is directed to a treatment for alumi num-magnesium alloys to prevent subsequent atmospheric and high-temperature oxidation and corrosion, and to facilitate degassing of finished and semi-finished articles of such alloys.

This application is a continuation-in-part of my copending application, Serial No. 684,641, filed September 18, 1957, now abandoned.

Thermal treatments, such as preheating for hot-working, annealing, solution heat-treatment and aging, are generally employed during the fabrication of products from aluminum and aluminum base alloys. These treatments are usually carried out in air atmosphere furnaces. In an air atmosphere which has not been dried and at temperatures above 800 F. but below the melting point of the alloy, aluminum-magnesium alloy articles tend to blister and stain, resulting in an objectionable appearance and sometimes a considerable decrease in mechanical strength. This effect is referred to as high-temperature oxidation.

Some finished and semi-finished articles of aluminummagnesium alloys have been found to contain appreciable. quantities of hydrogen, which may give rise to objectionable discontinuities in the metal structure. These discontinuities are generally quite easily detectable by ultrasonic testing techniques. It has been proposed to heat such articles in air for prolonged periods of time to diffuse hydrogen from the metal into the surrounding atmosphere. However, it has been found that the presence of small quantities of moisture in the surrounding atmosphere results in the failure of such degassing. This failure to degas may be explained by reaction between the metal surface and moisture forming metal oxides and a high hydrogen partial pressure at the metal surface which prevents loss of hydrogen from the metal into the surrounding atmosphere. In some cases, the hydrogen partial pressure may be sufficiently high to cause additional gassing of the metal.

To minimize high temperature oxidation, various methods have been employed to protect these alloys at temperatures above 800 F. US. Patent 2,092,033 to Stroup discloses the addition of. fluorine-containing substances in vapor form to a non-dried air atmosphere to provide a protective environment for such alloys. The vapor is conveniently generated from solid compounds which decompose at the elevated temperatures, above about 800 F. The metal is usually charged to a furnace already heated to the desired high temperature and containing the protective atmosphere or supplied with the vapor-generating fluoride compounds it the protective atmosphere has not been previously provided, and it is quickly heated to the desired temperature.

US. Patent 2,379,467 to Abbe discloses. a protective treatment for aluminum alloy forgings in which an aqueous solution of sodium fluoroborate is. applied to ice the forging which is then dried prior to being heated to the solution heat treating temperature. The coated forging is quickly heated to the required temperature in accordance with conventional practice.

Although these processes have been helpful in many cases, they have not eliminated altogether the undesirable blistering and staining which often occurs on articles of aluminum-magnesium alloys, nor have they proven effective in reducing the gas content of the metal.

It has been found that the blistering and staining of such alloys is apparently related to the oxidation of magnesium atoms at the external surface of the article and that the particles of these oxides or other oxy-compounds act as focal points or channels for further oxidation on and within the surface. This oxidation is apparently caused by the attack of water upon the surface, oxidizing the aluminum and magnesium and releasing hydrogen.

It has further been determined that above a certain critical temperature, there is a rapid increase in the rate of magnesium atom diffusion to the surface and concomitant oxidation of the magnesium. Whereas in previous methods oxidation has been inhibited by providing a protective atmosphere above that critical temperature, it has now been discovered that, for complete protection of aluminum-magnesium alloys, they must be subjected to preliminary vapor treatment below that temperature, more particularly, at a point below the onset of any substantial amount of magnesium oxidation.

It is an object of this invention to provide a preliminary thermal treatment for aluminum-magnesium alloy articles which substantially prevents high-temperature oxidation.

Another object is to provide a treatment for aluminummagnesium alloy articles which will promote subsequent degassing of such articles in an atmosphere containing moisture.

It is also an object to provide a treatment for such articles which develops a surface condition that permits storage in industrial atmospheres for prolonged periods of time without corrosion.

A further object is to provide surface protection on aluminum-magnesium alloy articles which is stable at elevated temperatures in the presence of oxygen and moisture.

It has been discovered that the foregoing objects can be achieved by subjecting finished or semi-finished articles of the aluminum base alloys containing magnesium to a preliminary thermal treatment wherein they are exposed to an air or inert gas atmosphere containing at least milligrams and preferably above 270 milligrams of boron trifluoride per cubic foot of furnace atmosphere for a period of about one to forty-five minutes at a temperature of 400 to 750 F., but in all cases below the critical oxidation temperature of the alloy. Generally, it is preferred to heat the article to a temperature of above 400 F. but below 575 F., the lowest critical oxidation temperature observed for an aluminum-magnesium alloy. Although the atmosphere may consist substantially entirely of boron trifluoride vapors, yet for practical purposes it is preferred to use not more than 30 grams per cubic foot of furnace atmosphere so as to minimize problems of handling in commercial operations. The alloy articles which have been treated in this manner strongly resist oxidation at temperatures above 800 F. and up to the melting point of the lowest melting point constituent in the alloy which is often referred to as the temperature of incipient fusion. In addition, the treated articles may be degassed 'by subsequent heating for prolonged periods of times, even in the presence of moisture.

The criticalox-ida-tion temperature of an aluminummagnesium alloy is defined as the lowest temperature at which the formation of magnesium oxide can be detected by electron diffraction techniques. This may be a particular temperature or a very narrow temperature range on the order of 20 F., and hence the term is intended to cover both conditions. It is directly dependent upon the length of exposure in an untreated air atmosphere, the humidity of the furnace atmosphere, and the alloy composition, primarily the magnesium content. Although it can be determined readily for each set of conditions, it has been observed that these points fall within the range of 575 to 750 F., and generally 600 to 675 F.

Generally, the electron diffraction determination involves heating samples in air to various temperatures and under various conditions of humidity and time. After such treatment, the samples are exposed to an electron beam in diffraction apparatus wherein the beam is reflected from the surface of the samples and a pattern is made upon a film as in X-ray diffraction studies. By comparing the pattern with that of a known sample, the nature of the substances on the surface of the metal sample can be determined. This diffraction pattern provides a qualitative indication of the presence of magnesium oxide.

The term aluminum-magnesium alloy as herein employed refers to an aluminum base alloy containing on a weight basis 0.1 to percent magnesium, with or without the presence of other elements, such as from 0.1 to 12 percent copper or 0.25 to 14 percent silicon or 0.1 to percent zinc or 0.1 to 3 percent manganese, or combinations of two or more of these elements. Any of the foregoing alloys may also contain one or more of the following elements, often referred to as hardeners, in the following percentages:

0.05 to 0.5 percent chromium 0.01 to 0.5 percent titanium 0.25 to 2.5 percent nickel 0.01 to 0.5 percent boron 0.002 to 2 percent beryllium 0.1 to 0.5 percent molybdenum 0.1 to 0.5 percent zirconium 0.1 to 0.5 percent tantalum 0.1 to 0.5 percent colombium 0.1 to 0.5 percent cobalt However, the total amount of the latter elements should not exceed about 3 percent.

The term aluminum base alloy as used herein refers to those compositions which contain at least 50 percent by weight of aluminum.

The boron trifiuoride-containing atmosphere may be provided by two methods. Solid inorganic carrier compounds may be deposited in a conventional air atmosphere furnace which, when heated to a temperature below the critical oxidation temperature of the alloy, will decompose or volatilize and yield boron trifiuoride. As an alternative, the boron trifiuoride vapor from a suitable source may be injected into the furnace atmosphere which has been heated to the desired temperature.

The inorganic carrier compounds should not yield the boron trifiuoride below about 400 F. since the protective film, if any, developed below this temperature is considerably less effective, possibly due to a difierent crystalline structure in the protective surface. The term carrier as used herein refers to those compounds which contain boron trifiuoride in chemically combined or sorbed form but yield the free fluoride when heated above 400 F. but below 750 F. Examples of compounds found suitable as carriers are ammonium fluoroborate and calcium fluoroborate.

The above-described methods and compounds may be used singly or in combination.

The alloy articles are preferably exposed to the boron trifiuoride-containing atmosphere for a period from one to forty-five minutes within the prescribed temperature range. Treatments of less than one minute do not permit development of a satisfactory protective film while treatments in excess of forty-five minutes do not provide any additional protection or oifer any further benefit. Generally periods of five to fifteen minutes are sufficient; however, longer times of treatment are desirable in the case of alloys particularly susceptible to oxidation or when the surface oxide film is of greater thickness than that normally developed by exposure to the atmosphere.

The present process may be practiced in an untreated air atmosphere, i.e., a normal air atmosphere as commonly employed in heat-treating furnaces. No drying of the air need be undertaken as moisture can be tolerated; in fact, the invention has been successfully employed even in furnace atmospheres having dew points on the order of 125 F. Also, observations indicate that more satisfactory results are obtained when the furnace atmosphere contains at least grain of water per cubic foot of furnace volume, but it should not exceed about 45 grains per cubic foot. In addition, the atmosphere may be contaminated with such substances as sulfur dioxide, hydrogen chloride, ammonia, carbon monoxide and carbon dioxide without adverse effect upon the action of the boron trifiuoride.

Gases which are inert toward the aluminum-magnesium alloys may be employed in place of air such as nitrogen, argon, helium and fuel gas. The term atmosphere as used herein includes air, the inert gases, or combinations thereof.

Further, this invention is most effectively practiced when the alloy articles have not previously been subjected to temperatures above their critical oxidation temperature. The presence of magnesium oxide on the surface, which has been produced in preceding thermal treatments, is observed to reduce the effectiveness of boron trifiuoride, presumably due to the formation of oxyfluoride complexes, but there is still substantial benefit to be derived from the treatment of the present invention.

When the inorganic boron-trifiuoride carrier compounds are utilized, it is desirable to spread out the substance in a thin layer within the furnace so that a large surface area is exposed to heat and decomposition of the entire quantity may take place more readily.

The temperature employed for the degassing treatment should be above 750 F. but below the temperature of incipient fusion of the alloy. As is well known, the higher the temperature the greater the rate of diffusion and the shorter the time required. Generally, for aluminum-magnesium alloys a temperature of about 900 to 975 F. has been found satisfactory.

The period for the degassing treatment will depend primarily on the thickness of the article being treated (the shortest diffusing path). Generally at 940 F. periods in excess of several hours and up to 20 days or more are necessary; for articles having a maximum cross-section of /2 inch, periods in excess of 16 hours have been found adequate. A thickness of 1 inch will generally require a period in excess of 40 hours at the same temperature; 10 to 15 days may be required for cross-sections thicker than 3 to 4 inches. Generally, the time required for the treatment will be related to the rate of hydrogen diffusion in the alloy at a given temperature.

Subsequent to the degassing treatment, the treated article should be subjected to a working step to effect some reduction in size, thus healing the voids left by the diffused hydrogen. The percentage of reduction necessary will be determined by the nature of the article and its original gaseous content.

The term degassing, as used herein, contemplates both the removal of gas from the metal and the prevention of regassing of the metal.

The efficacy of the present invention is illustrated by the following examples:

EXAMPLE 1 A lot of cold rolled sheets 0.064 inch in thickness of an alloy nominally composed of aluminum, 1.0 percent magnesium, 0.6 percent silicon, 0.3 percent copper, 0.25 percent chromium, and 0.06 percent titanium, was divided into two groups for test purposes. The first group was pretreated by exposure for a period of minutes at a temperature of 575 R, which is below critical oxidation emperature, to an undried air atmosphere containing boron trifluoride generated by the decomposition of ammonium fiuoroborate spread out on the floor of the furnace grams being used per cubic foot of furnace volume). Subsequent to the pretreatment this group of specimens was subdivided into three lots, each of which was heated at 1070 F. for a different period of time, namely, /2, 32 and 64 hours, in an air atmosphere having a, dew point of 125 F. The specimens were quenched in water at the conclusion of the heat treatment. The second group, which was not pretreated, also was divided into three lots and each one was heated at 1070 F. for the same periods of time as the specimens in the first group. These specimens were quenched in the same manner as those of the first group. All of the specimens of the first group were found to be free from blisters and staining, and metallographic examination indicated negligible traces of high temperature oxidation even after 64 hours. Such a long time of treatment is a severe test since it was much longer than the period of one to four hours normally employed for this alloy. Visual examination of the second group, the untreated specimens, indicated that those exposed for only /2 hour exhibited light blistering and a grey-gold stain, and that upon longer exposure the oxidation increased; after 32 hours there was heavy blistering and a grey stain, and at the end of 64 hours, there was very severe blistering and a grey-black stain. Metallographic examination of the untreated specimens indicated the high temperature oxidation effects were moderate after /2 hour but heavy. after 32 hours and very severe after 64 hours.

Specimens from each subdivision of the two groups were artificially aged at 350 F. for sixhours in accordance with commercial practice and their tensile properties determined. The results are indicated in Table I below:

Table I MECHANICAL PROPERTIES. OF TREATED AND UN- TREATED ALLOY SPECIMENS 1 Specimens not capable of being tested because-oi severe deterioration.

The diflierence in mechanical properties of the treated and untreated alloy specimens appearing in Table I is significant and provides further evidence of the protective value of the treatment in a. boron trifluoride containing atmosphere. High temperature oxidation brought about a complete loss in. the mechanical properties of the untreated alloy over the longest period of exposure. In the absence of such oxidation the mechanical properties remained essentially constant after an early decrease of small proportions which may be attributable to the extreme severity of the conditions to which they were exposed. The reduction in elongation of the untreated lots exposed 32 and 64 hours is especially significant because it is considered to reflect. rather closely the progress of penetration of the alloy by the undesired oxide. For this alloy in the artificially aged condition. 10% elongation is EXAMPLE 2 An alloy of the composition indicated in Example 1 and in the form of tube blooms was treated at 550 F. for a period of 30 minutes in an undried air atmosphere con taining boron trifluoi ide generated by the decomposition of ammonium fluoroborate (25 grams per cubic foot of furnace volume). Both the. treated and untreated specimens were exposed to an industrial atmosphere containing chlorine vapors for periods ranging from one to six weeks. Upon inspection, all of the treated blooms were f un to e. free. from, corrosion. regardless of length of exposure whereasthe untreated blooms showed a depth of attack of 0.0004 inch at the end of one week, 0.0008 inch at two weeks, 0.0010 inch at; four weeks, and 0.0028 inch at six weeks.

EXAMPLE 3 L-shaped extrusions of an alloy composed of aluminum, 2.5 percent magnesium, 5.75 percent zinc, 1.6 percent copper, 0.24 percent-- chromium, 0.05 percent titanium,

4 and 0.004 percent *berylliurn'were heated in a vertical air atmosphere solution heat-treating furnace to -a temperature 450 R, which is below the critical oxidation temperature, at which temperature three pounds of boron trifluoride gas was injected into the undried furnace atmosphere (.09 gram boron trifluoride per cubic foot of furnace atmosphere). The boron trifiuoride containing atmosphere was recirculated for about 30 minutes While the temperature of the furnace was being elevated, and exhausted at about 650- F. by replacement with air.

, The specimens were subsequentlyheated to a solution heat treating temperature of 880 F. for a period of one hour andquenched in water. Visual inspection indicated no blistering or staining and metallographic examination disclosed negligible high temperature oxidation. Untreated specimens of the same-alloy subjected to the above thermal treatment showed moderate blistering and a greyblack stain.

EXAMPLE 4 Foil samples 0.003 inch in thickness of an alloy composed of aluminum, 1.5 percent magnesium, 4.5 percent copper, and 0.65 percent, manganese were divided into two groups. The first group of specimens was pretreated by exposure for about 15 minutes at a temperature of about 550 F. to boron trifluoride vapors generated by the decomposition of calcium fiuorobonate (8 grams boron trifluoride per cubic footv furnace volume). The second groupv received no pretreatment. Both groups were heated to920 F. for one hour in an air atmosphere having a dew pointof F. Inspection of the first group of treated specimens revealed no blistering or staining but the second or untreated group exihibited moderate blistering and [had a grey-gold stain.

EXAMPLE 5 Cold rolled sheets 0.064 inch in thickness of an alloy of the composition indicatediin Example 1 were divided into two groups of specimens. The first group was pretreated by exposure for 15 minutes at a temperature of 500 F. in an undried air atmosphere containing boron trifluoride generated by the decomposition of ammonium fiuoroborate (10 grams boron tirifluoride per cubic foot of furnace volume) while the second group was not pretreated. The two groups. were divided into lots, placed in a furnace and each lot in one group was heated at a temperature of 960 F. for a particular period of time ranging from A to 64. hoursin length, as shown in Table 11-. All the specimens. were quenched in water from the. elevated temperature.

The specimens were subjected tometallographic examination and the degree or amount of high temperature oxidation was assigned an arbitrary numerical value from to 10, 0 representing no oxidation or only a negligible amount while 10 represented the most severe condition. The untreated specimens exposed 32 and 64 hours were not rated because of very severe deterioration beyond the arbitrary scale. The results of the metallographic examination are indicated in Table II below.

Table II METALLOGRAPHIO EXAMINATION 0F TREATED AND UNTREATED ALLOY SPECIMENS Time of Amount of Oxidation Exposure Hrs.)

Treated Untreated rampa e 10. Severe Deterioration.

EXAMPLE 6 A lot of forgings of an alloy composed of aluminum, 4.3 percent zinc, 3.3 percent magnesium, 0.60 percent copper, 0.2 percent manganese, 0.18 percent chromium and 0.06 percent titanium was divided into two groups. The forging weighed approximately 600 pounds and varied in thickness from about /2 inch to 3% inches. The first group containing 37 forgings was pretreated by exposure for about 15 minutes at a temperature of about 500 F. in an undried air atmosphere containing boron trifluoride generated by the decomposition of ammonium fluoroborate deposited within the furnace (1.31 grams of boron trifluoride per cubic foot of furnace volume). After pretreatment, the forgings were degassed by exposure to a temperature of 940 F. for 72 hours in an undried air atmosphere. The second group of 38 forgings was neither pretreated nor degassed.

Both groups were then subjected to a second blocking step and a finishing forging step, after which they were solution heat-treated at 830 F., quenched in water and subsequently precipitation hardened at 240 F. After heat treatment the forgings were ultrasonically inspected, the reject standard being an indication equal to or larger than that obtained from a No. Series B Alcoa Ultrasonic Standard Reference Block of the appropriate metal distance. The ultrasonic report on the first or treated group was as follows:

32. piecesclear (free from ultrasonic indications) lpiece1 (#3); 3 (#3+) 2 piece1 (#3); 4 (#3+); 2 (multiple #3) 1 piece1 (#5) 1piece1 (#5); 2 (#5+) or a total of 16 ultrasonic indications and a rejection of only two forgings. All of the forgings in the second group revealed ultrasonic indications and the group contained a total of 403 indications ranging from #3 to multiple #8+; from this group 22 forgings or 58 percent were rejected.

The diiference in the two groups indicated by the ultrasonic evaluation is most significant. Th'at thirty-two of the forging were completely free from indications is clearly convincing of the eflicacy of the present inventions in facilitating degassing of aluminum-magnesium alloy articles.

, 8 EXAMPLE 7 A lot of forgings of an alloy composed of aluminum, 4.4 percent copper, 0.8 percent silicon, 0.8 percent manganese and 0.4 percent magnesium was divided into two groups of 87 pieces each. The forging weighed about pounds and varied in thickness between about inch and 3 inches. The first group was pretreated by exposure for fifteen minutes at a temperature of about 500 F. in an undried air atmosphere containing boron trifluoride generated by the decomposition of ammonium fluoroborate (1.41 grams of boron trifluoride per cubic foot of furnace volume), and was then subjected to degassing at a temperature of 940 F. for about 72 hours in an undried air atmosphere. Both the degassed forgings and the untreated group were subjected to a blocking step and a finishing forging step after which they were solution heat-treated at 940 F., quenched in water, and then precipitation hardened at 340 F. Ultrasonic inspection indicated that the first group was substantially free from indications and all pieces passed inspection standards; the second group was found to contain numerous indications and 65 pieces were rejected for a recovery rate of only 25 per cent.

EXAMPLE 8 A lot of forgings of the alloy described in Example 7 and which weighed about pounds each and varied in thickness between about /2 inch and inch were divided into two groups. The first group was pretreated by exposure for about fifteen minutes at a temperature of about 500 F. in an undried air atmosphere containing boron trifiuoride generated by the decomposition of ammonium fiuoroborate (1.41 grams of boron trifluoride per cubic foot of furnace volume). The pretreated specimens were then subjected to degassing at a temperature of 940 F. for about 72 hours in an undried air atmosphere. Both the degassed forgings and the untreated group were subjected to a second blocking step and a finishing forging step, after which they were solution heat-treated at 940 F., quenched in water and precipitation hardened at 340 F. Upon inspection, the first group was found to be substantially free from ultrasonic indications and all pieces passed inspection standards while the second or untreated group contained numerous indications and only 50 percent of the forgings met acceptance standards.

Having thus described my invention, I claim:

1. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent magnesium comprising exposing said articles to an atmosphere containing at least 75 milligrams of boron trifluoride vapor per cubic foot of furnace atmosphere for a period from one to forty-five minutes at a temperature between 400 F. and 750 F., but below the critical oxidation temperature of said alloy.

2. The method in accordance with claim 1 wherein the atmosphere consists of air.

3. The method in accordance with claim 1 wherein the atmosphere consists of air containing at least but not more than 45 grains of water per cubic foot.

4. The method in accordance with claim 1 wherein the articles are heated to a temperature between 400 and 575 F.

5. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent by weight of magnesium comprising providing an atmosphere in a furnace substantially inert toward the alloy, placing said articles in said furnace, introducing boron trifluoride vapor into said atmosphere in an amount of not less than 75 mg. per cubic foot of atmosphere, and heating said articles to a temperature between 400 F. and 750 F. but below the critical oxidation temperature of said alloy for a period of one to forty-five minutes.

6. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent of magnesium comprising providing an atmosphere in a furnace substantially inert to ward the alloy and exposing a solid inorganic boron trifluoride-containing compound to the atmosphere which yields boron trifiuoride at a temperature between 400 F. and 750 F., sufficient compound being supplied to provide at least 75 mg. of boron trifluoride vapor per cubic foot of atmosphere, introducing said articles into said atmosphere in contact with said boron triiiuoride-containing compound and heating said articles to a temperature of at least 400 F. but below 750 F. and not higher than the critical oxidation temperature of said alloy for a period of one to forty-five minutes at which temperature the said inorganic compound yields boron trifiuon'de vapors.

7. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent magnesium to improve the resistance to corrosion at room temperature and resistance to oxidation at elevated temperatures comprising exposing said articles to an atmosphere containing at least milligrams of boron trifluoride vapor per cubic foot of furnace atmosphere for a period from one to fortyfive minutes at a temperature between 400 F. and 750 F., but below the critical oxidation temperature of said alloy.

8. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from (ll to 15 percent magnesium prior to degassing by thermal treatment at temperatures above 750 F. but below the temperature of incipient fusion comprising exposing said articles to an atmosphere containing at least 75 milligrams of boron trifluoride vapor per cubic foot of furnace atmosphere for a period from one to fortyfive minutes at a temperature between 400 F. and 750 F., but below the critical oxidation temperature of said alloy.

No references cited. 

1. THE METHOD OF TREATING FINISHED OR SEMI-FINISHED ARTICLES COMPOSED OF AN ALUMINUM BASE ALLOY CONTAINING FROM 0.1 TO 1K PERCENT MAGNESIUM COMPRISING EXPOSING SAID ARTICLES TO AN ATMOSPHERE CONTAINING AT LEAST 75 MILIGRAMS OF BORONN TRIFLUORIDE VAPOAR PER CUBIC FOOT OF FURNACE ATMOSPHERE FOR A PERIOD FROM ONE TO FORTY-FIVE MINUTES AT A TEMPERATURE BETWEEN 400*F.AND 750*F., BUAT BELOW THE CRITICAL OXIDATION TEMPERATURE OF SAID ALLOY. 